The present invention relates to sewage disposal. Currently, treated sewage, e.g. digested sludge, is commonly disposed of by simple dumping in shallow inshore waters, usually from barges or by direct pumping through pipelines from onshore sites. This is leading to an unacceptable build-up in the inshore environment of toxic substances such as heavy metal compounds.
According to the present invention, treated sewage such as digested sludge is loaded into the tanks of a large ocean going tanker [e.g. an oil/bulk ore (OBO) vessel, or a large, very large or ultra large crude carrier], transported therein to a deep water site, and then deposited directly onto the seabed at that site through piping deployed from the tanker and extending downwardly therefrom. The piping may be a flexible hose (e.g. plastics tubing in continuous or segmented form), or a string of steel pipes, or a combination of the two--in which case a hose portion descend from the vessel and connect with a pipe string extending the remainder of the way. The piping may be deployed over the side of the vessel, or from a moon pool. The depth of the seabed for deposit might for example be one thousand of fifteen hundred meters, or considerably deeper, e.g. 7000 m; in extremely deep water it may not be necessary for the piping to extend fully to the seabed, although it is recommended that it extend at least below the depth at which the majority of fish are found (the "fish line" ) and below the depth where there are significant thermal changes. At depth of about 4000 meters or more it is preferred to release the sewage sludge close to the ocean bed. The height of the point of release above the bed is a function of the relative densities of the sewage sludge and the sea water at the release depth, the release rate, and the buoyancy frequency of sea water. It is currently considered that an optimal release height in deep water is about 350 m above the abyssal plain.
Treated sewage as currently received for disposal is usually an aqueous digested sludge having a dry solids content of 10 to 5 wt.% or less; this may be concentrated, e.g. by partial de-watering by centrifuge or the like before or after loading onto the disposal tanker; such partially de-watered slurry may be re-diluted on board the tanker before seabed deposit if this should be necessary for ease of pumping. The treated sewage taken on board the disposal tanker may if desired be further treated on board before seabed disposal; the sewage may have added thereto various microorganisms to assist in breakdown and degradation of the deposited matter. The disposal tanker may be provided with an onboard laboratory for analysing sewage and also water samples before and after disposal.
The invention not only provides a disposal procedure as described above, but also the disposal tanker per se, provided with a hose reel and/or pipe erection plant and/or equipment for handling piping and for deploying the disposal piping from the tanker to the deep seabed disposal region. When piping is deployed over the side of the vessel the tanker hull may be used to provide a weathershield by allowing the tanker to be beam-on to wind and waves. Pipe erection plant e.g. pipe handling derricks may be accommodated within the hold space of the tanker, or may be on deck and shielded from the elements by some suitable superstructure or housing.